Defence chiefs have been forced into an embarrassing emergency retraction after secret information about the UK's nuclear-powered submarines was inadvertently published on a website.

A technical error meant sections of a Ministry of Defence report that appeared to have been blacked out could in fact be read by anyone who copied and pasted them into another document.

The offending paragraphs have now been properly covered up [PDF] but were reported to have included expert opinion about the fleet's ability to withstand a catastrophic accident.

Details of measures used by the US navy to protect its own nuclear submarines were also accidentally made available, according to the Daily Star Sunday newspaper, which alerted the MoD to the problem.

The Conservative MP Patrick Mercer, who served in the army, told the newspaper the information would be "hugely interesting" to Britain's enemies and its release "potentially catastrophic".

The document involved was an assessment drawn up by the head of the defence nuclear safety regulator, Commodore Andrew McFarlane, concerning options for the reactors in future submarines to replace the Trident fleet.

In sections released intentionally on the parliament website, it concluded that the existing reactors were "potentially vulnerable" to fatal accidents – leading ministers to suggest new versions would be used.

Some parts of the declassified document were redacted when it was published to protect sensitive information but the technique used meant the words were still electronically available underneath.

An MoD spokesman said: "The MoD is grateful to the journalist for bringing this matter to our attention.

"As soon as we were told about this, we took steps to ensure the document was removed from the public domain and replaced by a properly redacted version. We take nuclear security very seriously and we are doing everything possible to prevent a recurrence of this."